Microelectronic substrates are widely used in many consumer, industrial and other products. As used herein, microelectronic substrates include semiconductor substrates such as semiconductor wafers and integrated circuit chips, and also include nonsemiconductor microelectronic substrates such as second level packages for integrated circuit chips, and other nonsemiconductor microelectronic devices such as thin film read/write heads for magnetic disks, or display devices.
In fabricating microelectronic substrates, it is often necessary to pattern an image thereon. Photolithography is frequently used for patterning an image on a microelectronic substrate. In a microelectronic substrate imaging system and method, such as a photolithographic system and method, radiation is directed along a radiation path from a radiation source to a microelectronic substrate to be patterned. Typically, a mask is placed in the radiation path between the source and the microelectronic substrate. The mask includes a mask pattern of areas which are transparent and opaque to the radiation, so that the mask pattern impinges on the substrate. Typically, a photosensitive layer such as a resist is placed on the microelectronic substrate so that an image of the mask pattern is formed in the photosensitive layer. The image so formed may then be used for performing lithography or other processes on the underlying microelectronic substrate. Microelectronic imaging systems typically use optical radiation in the visible or ultraviolet spectrum, but may also use radiation in the x-ray or other spectra.
Conventional microelectronic imaging systems use a transmissive mask, such that the radiation source is on one side of the mask, the microelectronic substrate is on the opposite side of the mask, and radiation passes through the mask in the radiation path from the source to the substrate. It has also been proposed to design reflective-mask imaging systems wherein the radiation source and the microelectronic substrate are both on the same side of the mask and wherein radiation impinging on the mask from the radiation source is reflected from the mask to the microelectronic substrate. One such reflective-mask system is known as the "Markle-Dyson" optical projection system and is described in Grenville et al., Markle-Dyson Optics for 0.25 .mu.m Lithography and Beyond, J. Vac. Sci. Technol., B9, page 3108 (1991) and in Lee et al., Silicon on Quartz Reflective Masks for 0.25 .mu.m Microlithography, J. Vac. Sci. Technol., B9, page 3138 (1991) .
In order to produce high density microelectronic devices on microelectronic substrates, it is necessary to provide a high resolution microelectronic substrate imaging system. In particular, state of the art microelectronic devices are presently fabricated with sub-micron design rules. For future sub-micron technologies, higher resolution of microelectronic substrate imaging systems will be needed.
Improved resolution can be achieved through the use of lower wavelengths of radiation and/or a higher numerical aperture in the imaging system. Both of these techniques decrease the depth of focus which is available in the microelectronic imaging system. This tradeoff is quantitatively described by Rayleigh's formula which states that the depth of focus is proportional to the wavelength of the radiation used and inversely proportional to the square of the numerical aperture of the imaging system.
A certain amount of depth to focus is needed in high resolution microelectronic substrate imaging systems in order to overcome a number of inherent uncertainties in the imaging process. These inherent uncertainties include nonplanarity of the microelectronic substrate, nonplanar topography of already patterned layers on the surface of the microelectronic substrate, finite resist thickness and focusing uncertainty in the imaging system.
Many solutions have been proposed to increase the depth of focus in microelectronic imaging systems, each with their own set of advantages and drawbacks. For example, phase shifting masks have been proposed. Phase shifting masks can increase the depth of focus through the use of transparent features which introduce a phase delay in certain areas of the mask. Unfortunately, the automated design of phase shifting masks suitable for general pattern types appears to be an exceedingly difficult task. Phase shifting masks are described in the following publications: Improving Resolution in Photolithography with a Phase-Shifting Mask, M. D. Levenson et al., IEEE Transactions on Electron devices ED-29, 1828 (1982); The Phase Shifting Mask II: Imaging Simulations and Submicrometer Resist Exposures, M. D. Levenson et al., IEEE Transactions on Electron Devices, ED-31, 753 (1984); Optimization of Real Phase Mask Performance, F. M. Schellenberg et al., 11th Annual Symposium on Photomask Technology, Proceedings of the SPIE 1604, 274 (1992).
Another approach for increasing the depth of focus is referred to as "modified illumination". Modified illumination can increase the depth of focus by changing the spatial coherence properties of the radiation incident on the mask. One particular modified illumination scheme is described in a publication entitled New Imaging Technique for 64M-DRAM, N. Shiraishi et al., Optical/Laser Microlithography V. Proceedings of the SPIE 1674, pt. 2, 741 (1992).
Another approach for increasing depth of focus is referred to as "pupil plane filtering" or "apodization". This technique can be used to increase the depth to focus by filtering the angular spectrum of the projected image. Unfortunately, the increase in depth of focus is achieved at the expense of degrading contrast, and the technique requires a redesign of the imaging system to incorporate a pupil filter. Pupil plane filtering is described in the following publications: Spatial Filtering for Depth of Focus and Resolution Enhancement in Optical Lithography, H. Fukuda et al., J. Vac. Sci. Technol. B9, 3113 (1991); and Evaluation of Pupil Filtering in High NA I-line Lens, H. Fukuda et al., presented at the MicroProcess Conference, July 1993 (to be published).
Another technique for increasing the depth of focus uses multiple exposures at different focus levels. This technique, referred to as "focus latitude enhancement exposure" (FLEX) uses multiple, sequential exposures at different focus settings and at suitably selected doses. The multiple sequential exposures are superimposed and added in the resist. Unfortunately, since the multiple images are added in the resist on an intensity basis, a decrease in image contrast results. Production throughput may also decrease due to the need for multiple sequential exposures. The FLEX technique is described in the following publications: A New Method for Enhancing Focus Latitude in Optical Lithography: FLEX, H. Fukuda et al., IEEE Electron Device Letters EDL-8, 179 (1987); and Improvement of Defocus Tolerance in Half-Micron Optical Lithography by the Focus Latitude Enhancement Exposure Method: Simulation and Experiment; H. Fukuda, J. Vac. Sci. Technol. B7, 667 (1989).
Another technique has been used to reduce the depth of focus necessary because of the topography of the microelectronic substrate. This technique uses a "three-dimensional" or "topography compensated" mask. The three-dimensional mask includes a single mask pattern which follows the topography of the microelectronic substrate. Since the mask follows the substrate topography, the depth of field need not be increased to accommodate the topography. Topography compensated masks are described in Published European Patent Application No. 0 453 753 A2 to Hakey et al. entitled Method and Apparatus for Enhancing the Depth of Focus in Projection Lithography.
It has also been pointed out that for proximity printing systems, such as X-ray lithography systems, the non-zero thickness of the absorber material has a beneficial effect on the contrast and the exposure latitude. See Hector et al., Modeling and Experimental Verification of Illumination and Diffraction Effects on Image Quality in X-Ray Lithography, J. Vac. Sci. Technol. B9, page 3164 (1992) .
In summary, high resolution microelectronic substrate imaging systems and methods sacrifice depth of focus in order to achieve high resolution. Techniques have been devised to regain at least some of the lost depth of focus, but may introduce problems of their own. Accordingly, there is a need to increase the depth of focus in a high resolution microelectronic substrate imaging system and method, without requiring excessively complex mask design procedures, redesign of the projection system, or decreasing the contrast of the image to an unacceptable degree.